The present invention relates to an illumination optical system used in illumination devices for image-taking apparatuses.
The conventional optical system for illumination devices uses a reflector, which converges a luminous flux that emitted backward from a light source. Moreover, there is an optical system that the improvement of efficiency and the miniaturization thereof are attempted by arranging an optical member, which formed by a transparent body, in front of the light source, and by using a total reflection action in the optical member, as shown in Japanese Patent Laid-open Application No. H10-115852 and Japanese Patent Laid-open Application No. 2000-250102.
Thus, in the conventional optical system for lighting devices, a predetermined light distribution characteristic is obtained by irradiating an irradiation plane with the emerged light from the light source via optical members such as a prism, Fresnel lens or reflector, and by optimizing the shape of the optical members.
In recent years, the miniaturization of digital cameras and portable devices equipped with this kind of illumination device is rapidly advanced; thereby it becomes necessary to miniaturize the optical system for the illumination devices. On the other hand, it is important that the optical characteristics of the illumination device are improved in a given small space, and a lot of proposals have been made for this requirement.
As mentioned above, small and efficient optical systems are proposed, in which a prism is arranged on the illumination plane side with respect to the light source; the total reflection in the prism is used. However, the reflector is still necessary for covering the rear side of the light source, and it is difficult to evade a decrease of efficiency that is caused by using the reflector.
Generally, the reflectance of high reflective materials used for a reflector is only about 80%, and moreover, there is a problem that wrinkles or abrasions are formed on a surface of the reflector when manufacturing it in accordance with the miniaturization of the reflector. Therefore, only a lower reflectance than the original reflectance of the materials can be obtained in a state in which the reflector is installed in an actual product, and this is a big problem for making the illumination optical system.